1. Field of the Invention
The present invention relates to a method of detecting the residual capacity of a battery for use on an electric vehicle, and more particularly to a method of detecting the residual capacity of a battery for energizing an electric motor to propel an electric vehicle while the battery is being discharged, charged, and left to stand.
2. Description of the Related Art
Electric vehicles carry secondary batteries that are chargeable for reuse as a power supply for energizing motors for propelling the electric vehicles and various electric accessories.
The batteries on electric vehicles may be in a discharging condition in which they supply an electric current to the motors and electric accessories, a charging condition in which they are charged by an electric current supplied from a battery charger, and a standing mode in which they are neither being discharged nor charged but the electric vehicles are left to stand.
For detecting the residual capacity of a battery on an electric vehicle, it is customary to integrate discharge currents at given periods of time, thereby calculating an integrated discharge quantity when the battery is being discharged, either subtract the integrated discharge quantity from a residual capacity of the battery immediately before the battery starts being discharged or integrate charging currents at given periods of time when the battery is charged, thereby calculating an integrated charge quantity, and adding the integrated charge quantity to the residual capacity of the battery immediately before the battery starts being discharged. The calculated residual capacity of the battery is displayed on a residual capacity indicator on the front instrumental panel or the like of the electric vehicle.
The driver of the electric vehicle sees the displayed residual capacity of the battery, and charges the battery with a battery charger before the residual capacity indicator points to an indication "0", i.e., an empty level.
According to the above conventional process of calculating the residual capacity of a battery, the calculated residual capacity tends to differ from the actual residual capacity of the battery because of a detection error of the charging current which is supplied to the battery when the battery is charged and a detection error of the discharge current which flows from the battery when the battery is discharged.
Another problem is that it is difficult to detect the residual capacity of a battery with high accuracy because the residual capacity varies depending on the temperature of the battery, the aging of the battery, and the discharge current per unit time.
If the residual capacity indicator displays a residual capacity higher than an actual residual capacity, then the driver may be liable to determine a time to charge the battery in error. For example, the actual residual capacity may reach the empty level even though the residual capacity indicator does not point to the empty level. When this happens, the speed of travel of the electric vehicle suddenly drops, and the electric vehicle is no longer able to run normally.